Does the Apple store replace the back glass of the iPhone XR?

The back glass is fused on the phone, so Apple doesn't replace the glass. Instead, they replace the phone. If you have AppleCare on the phone, the cost to replace is much less than if you don't have AppleCare.

Check out the pricing in this link --> iPhone Repair - Official Apple Support

Apple doesn't repair the back glass and the device must be replaced.

We here are user volunteers - most (if not all) will not have any personal experience with this particular issue

You will need to ask Apple directly > Contact Support

No. The entire device will have to be replaced.

Yes, if you pay for it. It is considered accidental damage. It could very well be like 399$ or something to fix. You might want to ask at an Apple Store.